Method for manufacturing semiconductor devices

ABSTRACT

A method for manufacturing semiconductor devices includes a step of etching a sample including an interlayer insulating layer containing Al 2 O 3  and a polysilicon or SiO 2  layer in contact with the interlayer insulating layer using a plasma etching system. The interlayer insulating layer is etched with a gas mixture containing BCl 3 , Ar, and CH 4  or He. The gas mixture further contains Cl 2 . The interlayer insulating layer is etched in such a manner that a time-modulated high-frequency bias voltage is applied to the sample. The interlayer insulating layer is etched in such a manner that the sample is maintained at a temperature of 100° C. to 200° C. The interlayer insulating layer and the polysilicon or SiO 2  layer are separately etched in different chambers.

The present application is based on and claims priority of Japanese patent application No. 2005-216084 filed on Jul. 26, 2005, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing semiconductor devices including alumina (Al₂O₃) layers and polysilicon or silicon dioxide (SiO₂) layers placed thereunder. The method includes steps of selectively processing these layers.

2. Description of the Related Art

Al₂O₃ that is a high-K material is used to manufacture fine semiconductor devices. In particular, Al₂O₃ is used to form insulating layers placed between control gates and floating gates included in flash devices. The two types of gates are made of polysilicon and isolated from each other. In order to manufacture the flash devices, polysilicon layers for forming the control and floating gates and the Al₂O₃ insulating layers are selectively etched because there are steps due to the floating gates and isolation.

Known insulating layers placed between the control gates and the floating gates have an oxide-nitride-oxide (ONO) structure and are etched with CF gas. Since high-integrated, high-speed devices have been demanded, insulating layers having a high dielectric constant are necessary; hence, a high-K material is used to form such insulating layers.

With reference to FIG. 4, a wafer (sample) for manufacturing flash devices includes a silicon substrate 206 having isolation trenches 207 filled with SiO₂; base insulating layers 205, arranged on the silicon substrate 206, containing SiO₂; first polysilicon layers 204, each placed on the corresponding base insulating layers 205, for forming floating gates; an interlayer insulating layer 203, placed over the first polysilicon layers 204, containing Al₂O₃; a second polysilicon layer 202, placed on the interlayer insulating layer 203, for forming control gates; and a hard mask 201 placed on the second polysilicon layer 202. The floating gates are formed in such a manner that the first polysilicon layers 204 are etched such that the isolation trenches 207 and the base insulating layers 205 are exposed.

As shown in a sectional view taken along the line A-A in FIG. 4, portions of the second polysilicon layer 202 each make contact with corresponding sections of the interlayer insulating layer 203 that are located on the isolation trenches 207. As shown in a sectional view taken along the line B-B in FIG. 4, other portions of the second polysilicon layer 202 each make contact with corresponding sections of the interlayer insulating layer 203 that lie over the base insulating layers 205 and the first polysilicon layers 204.

Therefore, the second polysilicon layer 202 and the interlayer insulating layer 203 containing Al₂O₃ must be selectively etched as shown in the sectional view taken along the line B-B.

For example, Japanese Unexamined Patent Application Publication No. 5-160084 discloses that gas containing BCl₃ is used to remove alumina deposits from contact holes.

Furthermore, Japanese Unexamined Patent Application Publication No. 2003-318371 discloses that PZT is etched with BCl₃ or Ar at high temperature such that the selectivity of a hard mask is increased.

Al₂O₃ is usually etched with gas containing Cl₂ or BCl₃. However, such gas has low selectivity for polysilicon and SiO₂. Therefore, if the gas is used, the following problem occurs: a problem that the first polysilicon layers 204 and the base insulating layers (gate oxide layers) 205 placed thereunder are etched during the removal of Al₂O₃ present around steps.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a technique for selectively etching Al₂O₃, polysilicon, or SiO₂.

In order to achieve the above object, a method for manufacturing semiconductor devices including Al₂O₃ layers and polysilicon or SiO₂ layers placed thereunder according to the present invention includes a step of the Al₂O₃ layers with a gas mixture containing BCl₃, Ar, and CH₄ or He.

In the method of the present invention, the gas mixture may further contain Cl₂ or may contain He instead of Ar.

In the etching step, a time-modulated bias voltage may be applied to the Al₂O₃ layers. The Al₂O₃ layers may be etched at a high temperature of 100° C. to 200° C. The Al₂O₃ layers and the polysilicon or SiO₂ layers may be separately etched in different chambers.

In order to achieve the above object, a method for manufacturing semiconductor devices according to the present invention includes a step of etching a sample including an interlayer insulating layer containing Al₂O₃ and a polysilicon or SiO₂ layer in contact with the interlayer insulating layer using a plasma etching system. The interlayer insulating layer is etched with a gas mixture containing BCl₃, Ar, and CH₄ or He. In the method, the gas mixture may further contain Cl₂.

In this method of the present invention, the interlayer insulating layer may be etched in such a manner that a time-modulated high-frequency bias voltage is applied to the sample. Alternatively, the interlayer insulating layer may be etched in such a manner that the sample is maintained at a temperature of 100° C. to 200° C. The interlayer insulating layer and the polysilicon or SiO₂ layer may be separately etched in different chambers.

According to the present invention, since the interlayer insulating layer containing Al₂O₃ is etched with the gas mixture containing BCl₃, Ar, and CH₄, the interlayer insulating layer can be partly removed such that most of the polysilicon or SiO₂ layer remains.

Since the gas mixture is used to etch the interlayer insulating layer, a large amount of deposits are formed on walls of the etched interlayer insulating layer and the walls are likely to be tapered. However, the amount of the deposits can be reduced and the shape of the etched interlayer insulating layer can be improved without sacrificing the etching selectivity of Al₂O₃ to polysilicon or SiO₂ in such a manner that the sample is maintained at high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a configuration of a plasma etching system suitable for a method according to the present invention;

FIG. 2 includes sectional views showing steps included in the method of the present invention;

FIG. 3 is a graph showing the relationship between the content of CH₄ in an etching gas used in the method of the present invention and the etching selectivity of alumina and also showing the effects of time-modulated bias voltages; and

FIG. 4 is a sectional view showing a configuration of a wafer, including an interlayer insulating layer containing alumina, for manufacturing flash devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for manufacturing semiconductor devices by etching a sample including an Al₂O₃ interlayer insulating layer and polysilicon or SiO₂ layers in contact with the Al₂O₃ interlayer insulating layer using a plasma etching system and a gas mixture containing BCl₃, Ar, and CH₄ or He.

The method according to the present invention will now be described. A plasma-generating section included in the plasma etching system is described in detail with reference to FIG. 1. In order to generate plasma, UHF waves and a magnetic field are used. The UHF waves are transmitted from a UHF power supply and reach an etching chamber through an antenna 101 and then a UHF wave-transmitting plate 102. The resulting UHF waves induce electron cyclotron resonance (ECR) under the action of the magnetic field generated from a solenoid coil 103 surrounding the etching chamber, thereby generating high-density plasma from process gas in the etching chamber.

After the high-density plasma is generated, a wafer (sample) 105 to be processed is provided on an etching table 107 to which a direct voltage is applied from an electrostatic attraction power supply 108, whereby the wafer 105 is electrostatically attracted onto the etching table 107. The etching table 107 is connected to a high-frequency power supply 106. Voltages are applied to ions present in the high-density plasma by applying a high-frequency bias voltage to the etching table 107 from the high-frequency power supply 106 such that the resulting ions are accelerated downward, that is, toward the wafer, whereby etching is started.

During etching, the pressure in the etching chamber can be controlled with an evacuation unit including a vacuum pump, a turbomolecular pump, and a variable valve placed between the turbomolecular pump and the etching chamber.

The method according to the present invention is further described with reference to FIG. 2. FIG. 2 is an illustration showing steps of processing the wafer 105. FIG. 2 includes sectional views A, arranged in the left column, taken along the line A-A of FIG. 4 and sectional views B, arranged in the right column, taken along the line B-B of FIG. 4. The wafer 105, shown in FIG. 4, to be etched includes a patterned hard mask 201; a second polysilicon layer 202 for forming control gates; an interlayer insulating layer 203 containing Al₂O₃; first polysilicon layers 204 for forming floating gates; a base insulating layers 205 containing SiO₂; and a silicon substrate 206 having isolation trenches 207 filled with, for example, SiO₂ as shown in FIG. 2A, these components being arranged in that order from the top.

As shown in FIG. 2B, the second polysilicon layer 202 is etched through the hard mask 201 with the plasma etching system shown in FIG. 1 using a gas mixture containing Cl₂, HBr, and O₂. Portions of the second polysilicon layer 202 have a small thickness as shown in sectional views B but other portions of the second polysilicon layer 202 have a large thickness as shown in sectional views A. Therefore, the second polysilicon layer 202 must be etched more selectively as compared to the interlayer insulating layer 203.

As shown in FIG. 2C, the interlayer insulating layer 203 is etched using a gas mixture containing BCl₃, CH₄, and Ar. In this step, the etching selectivity of Al₂O₃ must be greater than that of polysilicon or SiO₂.

As shown in FIG. 2D, the first polysilicon layers 204 are etched a gas mixture containing Cl₂, HBr, and O₂ and then overetched using a gas mixture containing HBr and O₂.

In the step shown in FIG. 2C, BCl₃ is used such that Al atoms are etched off with chlorine (Cl₂) and the Al—O bond of Al₂O₃ is broken with B.

When only BCl₃ is used, the etching rate of polysilicon is two times that of Al₂O₃ and is 1.5 times that of SiO₂.

When etching is performed at a pressure of 0.8 Pa using a gas mixture containing 30% of BCl₃ and 70% of Ar, the etching rate of Al₂O₃ is 29.8 nm/min, that of polysilicon is 44.9 nm/min, and that of SiO₂ is 37.3 nm/min.

In the method of the present invention, a gas mixture containing BCl₃, Ar, and CH₄ is used. This gas mixture has, for example, a BCl₃ content of 30%, an Ar content of 66%, and a CH₄ content of 4%. Since this gas mixture contains Ar and CH₄ in addition to BCl₃, the etching rates of Al₂O₃, polysilicon, and SiO₂ are reduced. However, a decrease in the etching rate of polysilicon is greater than a decrease in the etching rate of Al₂O₃, that is, the etching selectivity of Al₂O₃ is higher than that of polysilicon.

FIG. 3 is a graph showing the relationship between the content of CH₄ in an etching gas and the etching selectivity of Al₂O₃ to polysilicon or SiO₂.

With reference to FIG. 3, when the content of BCl₃ in the etching gas is 30% and the content of CH₄ is 2.7%, 3.4%, or 3.6%, the etching selectivity of Al₂O₃ to polysilicon is 0.7, 0.8, or 1.5, respectively, and the etching selectivity of Al₂O₃ to SiO₂ is 0.8, 0.9, or 1.0, respectively.

When the CH₄ content is 3.6%, the etching selectivity of Al₂O₃ to polysilicon is high and is substantially equal to the etching selectivity of Al₂O₃ to SiO₂. That is, when the CH₄ content is increased 2.7% to 3.6%, the etching selectivity of Al₂O₃ to polysilicon or SiO₂ is increased.

The etching selectivity of Al₂O₃ to polysilicon and that to SiO₂ can be increased by applying a time-modulated RF bias voltage to a sample. Conditions for applying such a voltage to the sample are as follows: a bias frequency of 400 kHz, an output of 50 W, an application time of 5×10⁻⁴ second, and a non-application time of 5×10⁻⁴ second. A bias voltage that is not time-modulated is continuously applied to the sample.

As is clear from FIG. 3, in the case that the CH₄ content is 3.6% and a RF bias voltage is used under the above conditions, the etching selectivity of Al₂O₃ to polysilicon is 2.9 or 1.5 when a time-modulated RF bias voltage or a non-time-modulated RF bias voltage, respectively, is used. Furthermore, the etching selectivity of Al₂O₃ to SiO₂ is 1.3 or 1.0 when the RF bias voltage is time-modulated or not time-modulated, respectively. That is, the etching selectivity of Al₂O₃ to polysilicon or SiO₂ can be greatly increased using such a time-modulated RF bias voltage.

When etching is performed under the above conditions, a large amount of hydrocarbons are deposited on walls of etched portions; hence the etched portions have a tapered shape. This can cause problems in subsequent etching steps. Such problems due to the shape of the etched portions can be solved without sacrificing the etching selectivity in such a manner that the amount of such hydrocarbon deposits is reduced by etching a wafer at high temperature, whereby the wafer can be vertically etched.

The effect of temperature on the shape of etched portions will now be described. When the temperature of the wafer is 50° C. or 150° C., the etching selectivity of Al₂O₃ to polysilicon is 0.8 or 1.0, respectively, and the etching selectivity of Al₂O₃ to SiO₂ is 0.8 or 0.9, respectively. When the wafer temperature is 50° C., the tilt angle is 50 degrees. On the other hand, when the wafer temperature is 150° C., the tilt angle is substantially square, 88 degrees. Accordingly, the tilt angle can be maintained substantially square without sacrificing the etching selectivity of Al₂O₃ to polysilicon or SiO₂ in such a manner that the wafer temperature is maintained at 150° C. during etching.

As described above, although the second polysilicon layer 202 and the first polysilicon layers 204 may be etched at a low temperature of 50° C. or a high temperature of 100° C. to 200° C., the interlayer insulating layer 203 is preferably etched at a high temperature of 100° C. to 200° C. in order to maintain the tilt angle square. Therefore, the first and second polysilicon layers 204 and 202 and the interlayer insulating layer 203 are preferably etched separately in different chambers, whereby satisfactory etching results can be achieved. 

1. A method for manufacturing semiconductor devices comprising: a step of etching a sample including an interlayer insulating layer containing Al₂O₃ and a polysilicon or SiO₂ layer in contact with the interlayer insulating layer using a plasma etching system, wherein the interlayer insulating layer is etched with a gas mixture containing BCl₃, Ar, and CH₄ or He, wherein the interlayer insulating layer is etched in such a manner that the sample is maintained at a temperature of 100° C. to 200° C., and wherein the interlayer insulating layer and the polysilicon or SiO₂ layer are separately etched in different chambers.
 2. The method according to claim 1, wherein in said step of etching the Al₂O₃ is selectively etched relative to etching of polysilicon and SiO₂.
 3. The method according to claim 1, wherein the interlayer insulating layer is etched in such a manner that a time-modulated high-frequency bias voltage is applied to the sample.
 4. The method according to claim 1, wherein the interlayer insulating layer is etched in such a manner that the sample is maintained at a temperature of 100° C. to 200° C. 